We’re back with a USMLE® Step 2 CK Question of the Day! Today’s case involves a 65-year-old woman in the emergency department, presenting with double vision, headache, and escalating symptoms. With a history of hypertension and amlodipine use, ophthalmologic examination uncovers bilateral papilledema and right eye medial deviation. Can you identify the most likely cause of her distressing double vision in this compelling medical scenario?
A 65-year-old woman comes to the emergency department complaining of double vision and headache, accompanied by nausea and vomiting. Her symptoms started three days ago and have worsened since the onset. Past medical history is significant for hypertension, for which she takes amlodipine. Temperature is 37.0°C (98.6°F), pulse is 90/min, respirations are 15/min, and blood pressure is 110/75 mmHg. Ophthalmologic examination reveals bilateral papilledema and medial deviation of the right eye. Abduction of the right eye is impaired. All other eye movements are normal in both the eyes. Which of the following is the most likely cause of this patient’s double vision?
A. Compression of a cranial nerve by an aneurysm in the posterior communicating artery B. Stretching and injury to a cranial nerve over the petrous portion of the temporal bone
C. Thrombosis of the cavernous sinus
D. Occlusion of the posterior cerebral artery
E. Occlusion of the posterior inferior cerebellar artery
Scroll down for the correct answer!
The correct answer to today’s USMLE® Step 2 CK Question is…
B. Stretching and injury to a cranial nerve over the petrous portion of the temporal bone
Before we get to the Main Explanation, let’s see why the answer wasn’t A, C, D, or E. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
Today’s incorrect answers are…
A. Compression of a cranial nerve by an aneurysm in the posterior communicating artery
Incorrect: Aneurysm in the posterior communicating artery can compress the oculomotor nerve. This causes the eye to deviate down and out and the affected pupil to dilate (mydriasis). None of this is seen in this patient. Instead, this patient is most likely having CN VI palsy due to increased intracranial pressure.
C. Thrombosis of the cavernous sinus
Incorrect: Cavernous sinus thrombosis can cause CN VI palsy, but this condition is usually associated with proptosis, orbital swelling, and other cranial nerve palsies (e.g., CN III, CN IV).
D. Occlusion of the posterior cerebral artery
Incorrect: The posterior cerebral artery supplies the medial midbrain. Its occlusion causes Weber syndrome, which is characterized by oculomotor nerve dysfunction with associated contralateral hemiparesis, which are not seen in this patient.
E. Occlusion of the posterior inferior cerebellar artery
Incorrect: Occlusion of the posterior inferior cerebellar artery causes lateral medullary syndrome (Wallenberg syndrome), which is characterized by loss of the gag reflex, dysphagia, hoarseness, and loss of pain and temperature sensation of the face and the contralateral side of the body. This patient does not have Wallenberg syndrome.
Main Explanation
This patient is exhibiting signs and symptoms of increased intracranial pressure (ICP) including papilledema, nausea, vomiting and headache. The ophthalmologic findings of diplopia and dysfunction of lateral gaze of the right eye are concerning for abducens nerve (CN VI) palsy. This patient most likely has a space-occupying lesion that is stretching and injuring the CN VI for which imaging (e.g. MRI) is warranted.
The abducens nerve (CN VI) is a pure motor nerve that supplies the lateral rectus muscle, which is responsible for abducting the eye. It exits the midbrain at the level of pontomedullary junction, then crosses over the petrous apex of the temporal bone, where it enters the cavernous sinus. It then travels towards the lateral rectus muscle. CN VI palsy causes loss of lateral movement of the affected eye, leaving the action of the medial rectus muscle unopposed, which leads to medial deviation of the eye and resulting diplopia. Diplopia usually worsens when looking towards the side of the lesion and resolves when looking away from the lesion.
The most common cause of abducens nerve palsy is increased ICP, which can stretch the nerve as it crosses over the petrous apex. Common causes of increased ICP include space-occupying lesions (e.g., tumor, abscess). Other causes of CN VI palsy include head trauma, cavernous sinus thrombosis, demyelinating disorders, ischemic vascular disease and mastoiditis or otitis media, which may spread to the petrous portion of the temporal bone and damage the abducens nerve.
Major takeaway
Increased intracranial pressure can stretch and injure the abducens nerve (CN VI) as it passes over the petrous apex of the temporal bone. The resultant CN VI palsy causes lateral gaze dysfunction of the ipsilateral eye.
References
Graham C, Mohseni M. Abducens Nerve Palsy. [Updated 2021 Jul 18]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK482177/ ––––––––––––
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